The present invention relates in general to a polishing device and more particularly to a polishing sheet material for finely polishing and finishing the surface of a crystalline substrate, particularly a semiconductor silicon wafer, to obtain a highly polished mirror surface.
The polishing sheet material of the type described above has been generally made by the following two methods:
Japanese Patent Publication No. 40-20273 published Sept. 9, 1965 which corresponds to U.S. Pat. No. 3,284,274 issued Nov. 8, 1966, discloses one method in which a solution of polymeric material is applied directly over a supporting layer or substrate of a non-woven fabric and then subjected to wet coagulation to form a honeycomb-like cellular structure of the polymeric material on the supporting layer. Then, the thin surface skin of the exposed face of the thus formed cellular polymeric layer is abraded or cut away to open the internal cells and produce a poromeric suede-like layer.
The conventional method disclosed in the Japanese Patent Publication has a serious disadvantage in that irregularity in the thickness of the honeycomblike cellular polymeric layer is produced due to the irregularities of density and thickness in the non-woven fabric itself as well as irregularity in penetration of the solution of polymeric material applied thereon. Consequently, it has been difficult to successfully obtain a polishing sheet material with uniform distribution and flatness of the cellular polymeric layer when the surface skin of the cellular polymeric layer is abraded or cut away to form the suede-like layer.
In the other conventional method, a solution of polymeric material, e.g., polyurethane elastomer, is applied over an impermeable synthetic resin film and then subjected to wet coagulation to form a honeycomblike cellular structure of the polymeric layer on the synthetic resin film. The resin film is then stripped off or removed, and the thin surface skin of the exposed face of the cellular polymeric layer is abraded or cut away to form a poromeric suede-like layer. The poromeric layer is then adhered using a suitable adhesive to a supporting layer or substrate of a non-woven fabric to provide a polishing sheet material. The thus formed poromeric suede-like layer, which has more satisfactory flatness than that of the first-mentioned conventional method disclosed in the Japanese Patent Publication, however, has a disadvantage that a desired flatness is not obtained since the poromeric suede-like layer is adhered to the non-woven fabric of which irregularity of thickness is unavoidable.
When there is a lack of flatness in the polishing sheet material, the nap of the protruding surface portion is locally and deeply scraped, causing local damage or scratches generally called as "wormhole." Such wormholes generally shorten the life of the polishing sheet material.
Also, when the polishing sheet material is attached to a metal plate of a polishing machine, it has been necessary to apply a urethane or acrylic resin filler on the reverse side of the non-woven fabric substrate in order to prevent the penetration of polishing liquid slurry and to control the nap of the fiber of non-woven fabric, and then attach the polishing sheet material to the metal plate using a special, expensive adhesive tape with double surface. Otherwise, the nonwoven fabric would allow the polishing liquid slurry to penetrate rapidly to the adhering surface, causing the polishing sheet material to peel or rise from the adhering surface.
Furthermore, although a cooling device is generally installed in the polishing machine to remove the friction heat which is generated by polishing action, the non-woven fabric (which has a low heat conductivity and acts as a thermal insulator) can produce discoloration on the silicon wafer surface from the accumulated heat.